The purpose of this research is to study to what degree patients with Klinefelter syndrome suffer from dyslexia and behavioral and learning disabilities and if testosterone replacement therapy alters these disabilities. Hypogonadal patients with Klinefelter syndrome undergo detailed neuropsychological testing before and after testosterone treatment. Men enrolled in the study also have blood drawn for the establishment of lymphoblastoid cell lines, with the goal being to examine genetic differences in the X chromosome to see if they impact on the occurrence and severity of the neuropsychological disabilities. This study capitalizes on synergy between the parent GCRC at Harbor- UCLA, where the patients are recruited and tested, and the Cedars-Sinai GCRC, where the lymphoblastoid cell lines are established.